This invention relates primarily to the optical polishing, lapping or figuring of optical surfaces, and is particularly useful in the production of large mirrors which may be spherical or aspherical and may be of eccentric shape.
The well-established process which the invention enhances consists of grinding, polishing and figuring, with loose abrasive or abrasive particles in a softer matrix. A pad or lap is used to apply the abrasive to the workpiece. xe2x80x9cFiguringxe2x80x9d is continued polishing, applied differentially over the surface to produce very fine changes of surface height.
The xe2x80x9ctoolxe2x80x9d is part of the machine. It carries the pad which applies the abrasive to the workpiece. The diameter of this pad is conventionally referred to as the diameter of the tool. The process is usually wet. After some polishing, the workpiece is cleaned and optically tested. The optical test identifies parts of the surface which, although polished, are erroneously proud of the desired profile. Further polishing is applied preferentially in the proud areas. This can be achieved typically by varying the pressure or speed of the polishing pad. Numerous attempts to reduce the errors may be necessary and the process is iterative.
For surfaces which are not spherical (parts of spheres), iterative figuring is a lengthy process. Mechanisation of the former hand-craft has been attempted with some success (see below), but has not provided a technique with the versatility of hand-craft. Large and small tools have different problems.
On the one hand, the best quality is achieved with large tools. They generally conform to the desired aspherical shape of the surface. However, those tools are built and rebuilt for particular workpieces and are often operated in an expensive research and development context with access to the engineers who designed them.
On the other hand, small tools may more accurately simulate hand-polishing and are versatile. Unfortunately, if they operated automatically, they tend to create residual defects which are difficult to remove with the same tool. For example, the edges of the tool may create many ridges or grooves which are narrower than the tool itself. They will be visible in the optical test but attempting to remove them with the same tool may create a fresh set of similar defects, slightly moved. Changing the tool introduces further problems. Removing tool-induced features by applying gradually less work in each pass, or by using feathered-out strokes of the tool, is very slow. These problems, whilst not invalidating the process, greatly prolong the series of iterations and the general complexity.
U.S. Pat. No. 4,128,968 (1978) described an automatic polishing machine in which the effect of the edge of a small tool is reduced by local sub-motion of the tool. When the small tool is dithered in position, the centre of the contact area is continuously polished but the edge of the area is less polished. The xe2x80x9cremoval profilexe2x80x9d is specified as being circularly symmetrical. The tool has a pattern of motion which may be helical over the whole surface. The tool itself can rotate on a subsidiary axis which can be the local axis of the tool. Dithering is a method or pattern of operation of the whole tool which attempts to remove the errors it otherwise produces. It is outdated by software: the pattern of motion of the tool can be computed optimally to improve the particular work piece, rather than to produce an arbitrary circular removal profile, which is not necessarily what is required. Also, the dithering effect inevitably makes the locally polished area larger, which again is not necessarily required.
U.S. Pat. No. 5,157,878 (1992) describes a polishing tool consisting of a running tape pressed against the workpiece.
UK patent application 2 259 662 (1993) describes a machine applicable to complex aspherical spectacle lenses which resembles a multi-axis milling machine with an unspecified polishing head substituted for a cutting head.
The University of Arizona and University College London have described machines with a complex tool of a diameter approximately half or more the diameter of the workpiece. Essentially all the active area of the tool is available to contact with the workpiece at any instant. The tilt angle of the tool is defined by its contact with the workpiece. An example is published in ESO Conference and Workshop Proceedings No.42, pages 215-218, ESO Garching, Apr. 27-30 1992, xe2x80x9cThe Production of Highly Aspheric Secondary Mirrors Using Active Lapsxe2x80x9d by D. D. Walker et al.
The Zeiss company has described a machine with an elongated complex tool whose length is approximately half the diameter of the workpiece. As with the large tool described above, essentially all the active area of the tool is available to contact with the workpiece at any instant. The tilt angle of the tool is defined by its contact with the workpiece. In their Patent Application GB-A-2163076, the complex tool is substantially coextensive with the workpiece.
The diameter of the tool is typically less than 25 percent of the diameter of the workpiece.
The pressure distribution applied by the tool can be axially symmetrical.
The tool can be mounted on driven bearings which enable it to sweep over the workpiece in any desired pattern of motion at controllable speeds.
The tool can be mounted on a subsidiary motorised spindle which may be on an axis of symmetry of a tool. The tool may controllably rotate on this spindle as part of its polishing motion.
The workpiece may be mounted on a turntable which continuously rotates.
The tool can be activated to move over a fixed workpiece in a pattern which gives the same effective work as if the workpiece were rotating.